Update the error views.

[davidlesieur]

The django-jinja documentation indicates the view that is provided to use with Django's handler500 is broken since Django 2.2.

The reason is that Django's 40x error views are expected to receive an exception positional argument, while the 500 view doesn't take that argument.

Since all we need from the django-jinja error views is to specify the template name, I didn't see the value of implementing custom class-based views, especially now that the 500 view is different from the others. Thus, I have also simplified the code to reuse Django's default error views.

Fixes #234.